Summary:
Unfortunately we can't use -DNDEBUG yet since we don't properly exclude the test libraries/executables from the non-debug builds on non-MSVC platforms. Previously this was failing on Linux for every build type except `CMAKE_BUILD_TYPE=Debug`.

Reverts a48a62d5
Closes https://github.com/facebook/rocksdb/pull/2595

Differential Revision: D5436182

Pulled By: ajkr

fbshipit-source-id: 062f07cc9ce06a073b66054722b27bac1890dca3